A Missouri state senator is calling on Gov. Jay Nixon to consider budgeting money for a gas chamber, citing concerns about the state's ability to find an adequate execution drug.

Republican Sen. Kurt Schaefer of Columbia wrote to the Democratic governor on Thursday urging him to either include gas chamber funding in his fiscal year 2015 budget or propose an alternative method of execution. State statutes currently allow executions either by lethal drug or by gas, though a gas chamber no longer exists in the state.

Phone messages left with a Nixon spokesman were not returned.

Schaefer's letter cites reports that the anti-death penalty European Union could limit export of the popular anesthetic propofol if Missouri moves ahead with plans to use it in an execution on Oct. 23. Missouri has turned to propofol instead of a three-drug protocol because drugs used for lethal injection in the past are no longer being sold to prisons and corrections departments because drug makers did not want them used in executions.

Schaefer, a former assistant attorney general who tried death row cases, told The Associated Press that Missouri law requires the death penalty to be carried out when that punishment is rendered.

"I think if this is going to stay on the books, we have to have a mechanism to carry it out," Schaefer said. "Otherwise we're arbitrarily not enforcing the laws."

Schaefer, who announced last month that he will seek the Republican nomination for state attorney general in 2016, became the second key figure in the state to broach the possibility of a return to the gas chamber. Democratic Attorney General Chris Koster brought up the idea in July. Koster, through his spokeswoman, declined comment on Schaefer's letter to Nixon.

Missouri used gas to execute 38 men and one woman from 1938 to 1965. After a 24-year hiatus, the death penalty resumed in 1989. Since then, 68 men — all convicted murderers — have been put to death, all by lethal injection. But as concerns were raised in the courts about the lethal injection process, Missouri has carried out just two executions since 2005.

Previous executions by gas took place at the Missouri State Penitentiary in Jefferson City. Prisoners were moved out a decade ago and it is now a tourist attraction — complete with tours of the old gas chamber.

Rita Linhardt of Missourians for Alternatives to the Death Penalty said it would be a waste of taxpayer money to fund a new gas chamber.

"It's still on the statutes, but that doesn't mean it's in the best interest of the people of the state to go back to such a gruesome method," Linhardt said, noting that some deaths by gas were slow and inmates appeared to suffer.

No state uses the gas chamber as a primary means of execution, though Missouri, Arizona and Wyoming list it as a secondary option, according to the Death Penalty Information Center. The last time the gas chamber was used in the U.S. was in Arizona in 1999.

Propofol is used in about four-fifths of all anesthetic procedures in the U.S., according to the American Society of Anesthesiologists. Roughly 85 percent of the U.S. supply of propofol is made by the German company Fresenius Kabi. The European Union prohibits trade in goods that could be used for executions. The EU is reviewing whether to make propofol subject to export controls.

While not a complete ban, a spokesman for Fresenius Kabi said the move would drastically slow the movement of propofol to the U.S., potentially causing a shortage.

Missouri altered its execution protocol last year to use propofol, but the execution of convicted killer Allen Nicklasson on Oct. 23 will be the first since the change. An execution is also scheduled Nov. 20 for another convicted murderer, Joseph Franklin.

Propofol made headlines in 2009 when pop star Michael Jackson died of an overdose. Missouri is the only state to adopt propofol for executions.

The Missouri Department of Corrections has said it has enough propofol for the two planned executions and one more, with the last batch expiring in February 2015.